I just crashed 5 gallons of German Pilsner for lagering and I noticed that my airlock starsan level was significantly lower after cooling. I presume this is due to the liquid (beer) shrinking after a 20F drop in temp, but I am not sure how to prevent this or protect against it. I am not terribly concerned with starsan going into the carboy, but more that it can drop below the airlock level which essentially is opening up the carboy to the air in the chest freezer.

How do you handle shrinkage when crash cooling in a closed system?

[edit: clarified that i was talking about the beer volume shrinking not the starsan]

  • you could top off the airlock before crashing...
    – baka
    Sep 5, 2012 at 16:14
  • I did top it off. Am I the only one who has this problem? Surely not.
    – Jason V
    Sep 5, 2012 at 17:06
  • 2
    It may be due to the cooling air contracting causing air to be pulled in, which might suck some of the starsan into the beer. But it's not shrinkage of the starsan itself - max shrinkage is only about 1/20 of the volume.
    – mdma
    Sep 5, 2012 at 18:19
  • mdma has it right. Its a simple function of pressure temp and volume. As it cools the head space pressure lessens creating a vacuum pulling in sanitizer from the airlock.
    – brewchez
    Sep 5, 2012 at 23:35

4 Answers 4


Don't bother with an airlock at that point. Just put some foil over the opening and hold it on with a rubber band. There's nothing outgassing at that point so there's no need for an airlock.

  • 1
    Occam's Razor wins...
    – Jason V
    Sep 5, 2012 at 20:20
  • 1
    It's called "Pragmatic Brewing"! :)
    – Denny Conn
    Sep 5, 2012 at 20:45

Any reduction in temperature will create a negative pressure (compared to what was before) in the fermentation vessel. The air lock is designed to work in a one-way fashion, so the only real risk is contamination of your fermentation chamber with air-lock fluid.

Use a vodka in the airlock instead of starsan. In the extremely rare event that you generate enough low pressure to suck in some of the airlock fluid, you are adding a generally tasteless grain alcohol to you brew (instead of a cleaning solution). In any event, the volumes are low enough that few people will ever be able to detect contamination should it ever occur.

Vodka is already high enough in alcohol content that it will prevent crud from growing in the airlock.


I've never had this problem. Here are some ideas:

The idea that you'd get enough shrinkage to really notice it sounds dubious to me though, seems more likely that it's leaking into your carboy or evaporating.

  • Not so dubious. The chilling effect of the liquid creates a vacuum and thats where the suck back is coming from. The OP is just calling it shrinkage. Overfilling the airlock will make it worse as more liquid will get sucked back into the airlock.
    – brewchez
    Sep 5, 2012 at 23:34
  • Yea, I thouught he was referring to the starsan shrinking due to temperature, which doesn't really make sense. Water expands slightly just before freezing. The question was edited to clarify that he was referring to the beer, but the vacuum from chilling the air and beer makes more sense.
    – paul
    Sep 6, 2012 at 3:25

I've had this problem. Once I cold-crashed a Better Bottle after plugging the opening with a solid stopper and the carboy puckered up. You can see the same effect if you put an opened bottle of warm soda in the fridge. I assume the air shrinks more than the liquid, but perhaps it's a combination.

It makes me a little uneasy, because once there is negative pressure in the carboy, air and contamination can slowly be drawn in. Even if you can get an airtight seal, as soon as you remove the stopper / blow-off tube, there's an inrush of air and oxygen.

My solution has been to transfer to a keg before I cold crash and slightly pressurize the keg with CO2. This has several benefits:

  1. There is enough pressure to resist the shrinkage.
  2. Beer is protected by CO2 for storage.
  3. It will speed up the carbonation process later on since there will already be some CO2 in solution.

I usually pressurize to 10 psi or so and then disconnect it. After a week or so the pressure has dropped almost to zero due to shrinkage and dissolution, but that's OK. You could also just leave it hooked up to the CO2 tank and carbonate it at the same time. Aging and carbonation don't interfere with each other.

  • Do you have problems with trub settling in the kegs and potentially clogging the dip tube?
    – Jason V
    Sep 5, 2012 at 19:39
  • @JasonV: No, I've never had the dip tube clog. I generally do longish primaries (2 or 3 weeks), so the beer is reasonably clear. The first pint or so after cold crashing is usually cloudy and I pour it down the drain.
    – Hank
    Sep 5, 2012 at 20:27

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